The ESP-01 .. ESP32 series of boards are a fantastic IoT platform.
Typically used with Arduino, since there is nothing out there that just blows it away.
IoT is not big boards like the Pi. It is small boards, limited RAM, limited CPU (the ESP8266 is 80MHHz though).
We do not need anything like Android for that. Not sure if something minix/Linux like would even work. For the slow CPUs (Arduino), interrupts are used for data, not context switch, and for timing, every single CPU cycle counts. Just like the old days.